The spectral characteristics of wide-bandgap semiconductors have been enhanced through the use of adsorbed dye molecules. This is referred to as dye sensitization, and cells incorporating these materials as photoanodes are described as dye-sensitized solar cells (hereafter DSSC's). See, for example, A. Hagfeldt and M. Graetzel, Acc. Chem. Res., 2000, 33, p. 269–277, which is incorporated herein by reference in its entirety. As a whole, DSSC's are composed of three basic components: the photoanode, the electron-transfer mediator, and the cathode. A rather large variety of semiconductors and dyes are known to yield effective photoanodes. There are, however, a limited number of known electron-transfer mediators and compatible cathode materials that produce effective solar cells when combined with a known photoanode. See, for example, Lenzmann, et al., J. Phys. Chem. B, 2001, 105, 6352; Magnisson et al., Solar Energy Materials and Solar Cells, 2002, 73, 51–58; and Turkovic et al., Solar Energy Materials and Solar Cells, 1997, 45, 275–281, all of which are incorporated herein by reference in their entirety.
Iodide salts of inorganic and organic cations when mixed with iodine (hereinafter I−/I3−) at varying ratios and concentrations in aprotic solvents are known to be effective electron-transfer mediators in DSSC's. See, for example, Nazeeruddin et al., J. Am. Chem. Soc., 1993, 115, p. 6382–90. Comparable bromide salts mixed with bromine (hereafter Br−/Br2) are less effective and less preferred, but are also known. See, for example, Heimer et al., J. Phys. Chem., 1993, 97, p. 11987–94; and Vlachopoulos et al., J. Am. Chem. Soc., 1988, 110, p. 1216–20. With both the I−/I3− and the Br−/Br2 mediators, there are a number of drawbacks. Often cell construction is complicated by issues of chemical compatibility with the electron-transfer mediator. Both the I−/I3− and Br−/Br2 mediator mixtures are highly corrosive. With a few exceptions, notably titanium or platinum, the use of metals must be avoided for successful long-term operation of the cells. The volatility of both iodine and bromine further complicates the sealing of cells, and leakage is often the cause of device failure.
There has also been a report of a cobalt complex-based mediator that rivaled the I−/I3− mediator in terms of the kinetics to regenerate the dye. Nusbaumer et al., J. Phys. Chem. B, 2001, 105, 10461. However, the ligands used to form this complex are not readily available, and are believed to require a multi-step synthetic procedure, thereby adding time and cost in producing these electron-transfer mediators. Many other previous efforts in cobalt complexes as mediators in DSSCs have also been not too successful. See, for example, Bonhote et al., Presented at the 10th International Conference on Photochemical Conversion and Storage of Solar Energy (IPS-10), Interlaken, Switzerland, 1994, Abstract C2; and Wen et al., Sol. Energy Mater. Sol. Cells, 2000, 61, 339.
Platinum and titanium are known to be compatible cathode materials, being resistant to the corrosive nature of the above electron-transfer mediators. Platinum is generally most preferred because its surface is known to be catalytic for the reduction of iodine to iodide. Gold, silver, nickel, iron, chromium, aluminum, and copper (along with most other metals and alloys thereof) cannot be used with I−/I3−. This limitation has been one of the major obstacles in the commercialization of DSSC's.
Passivation with electrically insulating materials of one or more of the active surfaces in the photoanode is known to allow the use of other electron-transfer mediators such as ferrocene/ferrocenium. See, for example, Gregg et al., J. Phys. Chem. B, 2001, 105, p. 1422–1429. However, the performance of these cells does not approach that of cells using I−/I3−. Additionally, the passivation methods are known to be difficult to control and reproduce.
The operation of the dye-sensitized photoanode in certain photoelectrochromic devices is subject to most of the same considerations as DSSC's. See, for example, Pichot et al., J. Electrochem. Soc., 1999, 146, p. 4324–4326, which is incorporated herein by reference in its entirety. Consequently, similar advantages will be accrued by replacing the I−/I3− electron-transfer mediator system with the present invention in photoelectrochromic devices.
Therefore, there is a need for other simple and efficient electron-transfer mediators.